Surprising seabirds: new species for Tristan da Cunha and Gough Island

Abstract

Seabirds are among the best studied organisms in the Southern Ocean. It was thus surprising to discover several 'new' species breeding at Tristan da Cunha and Gough Island over the last decade. There had long been confusion regarding the status of Fregetta storm petrels at these islands, with some authorities recognising White-bellied Storm Petrel F. grallaria leucogaster from the Tristan archipelago and a white-bellied form of Black-bellied Storm Petrel F. tropica melanoleuca from Gough Island, while others took the more pragmatic approach of calling them all White-bellied Storm Petrels. Blood samples collected in 2009 revealed the presence of both F. grallaria and F. tropica with white bellies occurring together on Inaccessible Island. Subsequent sampling in 2011 confirmed that both species were common the island, and identified some subtle morphological characters that allow the two species to be distinguished. As far as we can tell, only F. tropica melanoleuca occurs on Gough Island, where numbers of Fregetta storm petrels have been greatly reduced by mouse predation. Then in 2011/12, Karen Bourgeoise and Sylvain Dromzee noticed there were two different types of prions Pachyptila breeding on Gough Island: the well-known Broad-billed Prion P. vittata and another bird averaging 1-2% smaller in most dimensions, but 15% narrower in bill width. Bill width is particularly important for the larger whalebirds, because the number of filtering lamellae correlates with bill width. Remarkably, the presence of these two forms had been overlooked despite both being abundant on the island, and both breed within 1 km of the island's weather station! Subsequent work showed that the two forms breed roughly three months apart. Genetic comparisons with prions from throughout the Southern Ocean show that the new population on Gough is closely related to the relict population of P. macgillivrayi now confined to St Paul Island, and the extinct population of this species from Amsterdam Island. Both are sister to P. vittata, whereas Salvin's Prion P. salvini is sister to the Antarctic Prion P. desolata. Given the allochronous breeding of the two taxa on Gough Island, we advocate recognising Macgillivray's Prion as a full species, with a new subspecies P. macgillivrayi goughensis confined to Gough Island. The Gough subspecies represents >99% of the global population, and the species qualifies as Endangered because of suspected decreases resulting from very poor breeding success due to mouse predation. Another surprising finding occurred in 2014, when Blue Petrels Halobaena caerulea were discovered breeding on Gough Island. This is only the seventh island group where the species breeds, and is ~650 km farther north than the previous northern-most breeding site. Finally, on a more positive note, Common Diving Petrels Pelecanoides urinatrix were rediscovered breeding on Marion Island in 2015 for the first time since they were apparently eradicated by feral cats in the 1970s.